Field of the Invention
The present invention relates to an RhtB (homoserine/homoserine lactone export transporter) protein variant having an enhanced ability to export O-phosphoserine (OPS) that is a precursor of L-cysteine, a polynucleotide encoding the protein, a vector comprising the polynucleotide, an OPS-producing microorganism comprising the protein variant, a method of producing O-phosphoserine using the microorganism, and a method for preparing cysteine or its derivatives, which comprises reacting O-phosphoserine, produced by the OPS-producing method, with a sulfide in the presence of O-phosphoserine sulfhydrylase (OPSS) or a microorganism that expresses OPSS.
Description of the Prior Art
L-cysteine, an amino acid playing an important role in the metabolism of sulfur in all living organisms, is used not only in the synthesis of biological proteins such as hair keratin, glutathione, biotin, methionine, and other sulfur-containing metabolites, but also as a precursor for biosynthesis of coenzyme A.
Known methods of producing L-cysteine using microorganisms include a method of biologically converting D,L-ATC to L-cysteine using microorganisms (Ryu O H et al., Process Biochem., 32:201-209, 1997). Another known method is a method of producing L-cysteine by direct fermentation using E. coli (EP 0885962B; Wada M and Takagi H, Appl. Microbiol. Biochem., 73:48-54, 2006). Meanwhile, the present inventors found an enzyme (O-phosphoserine sulfhydrylase (OPSS)) that synthesizes L-cysteine from O-phosphoserine (OPS) in certain microorganisms. Based on this finding, the present inventors developed a method of producing cysteine by reacting OPS with the OPSS enzyme by culturing a mutated microorganism to accumulate OPS therein (Korean Patent Laid-Open Publication No. 10-2012-004111). The needs still exist to produce OPS in excessive amounts in order to produce cysteine at high yield. Accordingly, the present inventors have made extensive efforts to discover an appropriate exporter that enables O-phosphoserine produced in an OPS-producing strain to be released from the cells smoothly. In addition, based on various kinds of known transporters, the present inventors screened ydeD encoding O-acetylserine/cysteine efflux protein, yfiK encoding O-acetylserine/cysteine export permease (Franke I, Resch A, Dassler T, Maier T and Bock A, J. Bacteriology, 185: 1161-166, 2003), rhtB encoding homoserine/homoserine lactone efflux protein (Zakataeva N P, Aleshin V V, Tokmakova I L, Troshin P V, Livshits V A FEBS Lett 1999; 452(3); 228-32) and the like, and particularly found that the enhancement of RhtB in the OPS-producing strain results in an increase in the concentration of OPS (Korean Patent Laid-Open Publication No. 10-2012-0041115). However, for the production of higher yield of cysteine, the development of a transporter having a higher ability to export a precursor OPS from the OPS-producing strain is still required.